Cable networks were originally developed to facilitate the distribution of television broadcasts. Cable networks generally use coaxial cables as transmission lines for the dissemination of television broadcasts to various homes, businesses and the like. Cable networks were designed to support one-way communication of broadcast signals from a broadcast center to a number of television nodes connected to the cable network.
The “Internet” is a well-known term that refers to collection of data networks that utilize layered protocols to route data packets to various locations around the world. Internet carries and processes data traffic in internet protocol IP packets each of which contains source and destination IP addresses, and other control information, along with a segment of user data (sometimes referred to as the payload). As such, computers use the Internet to communicate data in IP packets and receive and process IP packets in accordance with the protocol.
The emergence of the Internet has prompted the desire to allow cable networks to support two-way packet-based data communication. This desire has resulted in standards and protocols that enable network infrastructure conventionally used for the distribution of cable television broadcasts to deliver Internet service to homes, businesses and the like. By adding the two-way packet based communication capability, cable networks can now support both one way communication of cable television broadcasts, and two-way packet based voice and data communication.
In order to allow a cable network to offer Internet access to cable subscribers in a bandwidth-efficient manner, the Cable Television Laboratories (CableLab) has developed a series of Data Over Cable System Interface Specification (DOCSIS) standards that govern the design of cable modems (CMs) residing in subscribers' premises, and of cable modem termination systems (CMTSs) deployed and managed by the cable operator. The network that interconnects the CMs to a CMTS is called the cable access network or DOCSIS network, which may physically consist of coaxial cables and optical fibers, and other transmission equipments, such as lasers and amplifiers, in the cable plant which is commonly referred to as Hybrid Fiber-Coax (HFC) network.
For a cable subscriber to communicate over the Internet, it uses a DOCSIS-compliant CM to interface with the DOCSIS network. As the gateway to the Internet, the CMTS controls the access of the CM to the bandwidth resources of the DOCSIS network. Although a coaxial cable has a usable RF spectrum in excess of 500 MHz, most of this spectrum is used for broadcasting TV programs. Accordingly, only a small portion is typically allocated to transport data via the DOCSIS network. Current versions of DOCSIS include DOCSIS 1.0, DOCSIS 1.1 and DOCSIS 2.0. Future releases of DOCSIS may continue to evolve to improve the two-way packet based communication and may entail more RF bandwidth for such services.
One challenge in DOCSIS and similar protocols, such as the wireless counterpart to DOCSIS that enables two-way packet based communication over a shared RF access medium, is the control and management of upstream communication. In this disclosure, the phrase “upstream communication” refers to communication from a CM to a CMTS. “Downstream communication” on the other hand, refers to communication from the CMTS to the CMs. A DOCSIS network may have several upstream channels and several downstream channels, each using a unique RF spectrum that does not conflict with other channels. The CMTS implements a scheduler to control each respective channel.
Controlling upstream communication is much more challenging than coordinating downstream communication because the CMs are independent from one another and compete for the upstream bandwidth. In contrast, the CMTS is generally the only device that transmits on the downstream channel. To facilitate upstream communication, the upstream scheduler dynamically assigns upstream time slots to individual CMs for transmission of their waiting packets. For the most part, the DOCSIS standards leave the design of the upstream scheduler flexible, i.e., the CMTS vendor is free to design its own upstream scheduler subject to certain general requirements. As a result, the quality of the upstream scheduler is a key determining factor for the upstream bandwidth efficiency of an CMTS.
Each CM is typically associated with one or more computers in a home or business that subscribes to Internet access through a DOCSIS network. Serving as a control interface between the CMs and the internet, the CMTS coordinates communication between various server computers attached to the Internet and subscriber computers equipped with the CMs. The CMTS directs data between the Internet and various CMs connected to the CMTS.
Upstream communication is supported in DOCSIS via a reservation-based time division multiple access (TDMA) protocol. In particular, the protocol requires that a CM make a time-slot reservation to transmit every single upstream data packet on the upstream channel. After obtaining a reservation, the CM can transmit the data packet during the reserved time slots. Such a reservation-based system can avoid data packets from being sent from different CMs at the same time, in which case the data packets sent from the different CMs would collide on the common upstream channel of the cable transmission line and thereby undermine the ability of the CMTS to receive the packets.
Central to the upstream time slot reservation mechanism is the “MAP” and the “MAP interval.” A MAP is an upstream channel control message broadcasted over the downstream channel by the CMTS on a regular basis in order to inform the CMs sharing a common channel of their reservation status and of the allocation of upstream time slots for the CMs to transmit time slot requests. In this disclosure, the term “TDMA map” refers to a MAP, or similar message in a non-DOCSIS system. Similarly the terms “map” and “map interval” refer respectively to a MAP or a MAP interval of a DOCSIS compliant system, or similar control messages of other reservation based TDMA packet transmission systems.
In a DOCSIS network, each MAP schedules a particular future time interval in which certain CMs may be granted time slots for their upstream packet transmissions. That future time internal is the “MAP interval” associated with the given MAP. The time (commonly known as the “Advance Time” in DOCSIS) between the time a MAP is transmitted on the downstream channel and the time when the MAP interval associated with the MAP begins, must be large enough to accommodate for transmission and propagation delays, as well as processing delays of the CMs. Two successive MAPs define back-to-back MAP intervals so that the upstream channel is covered by MAP intervals seamlessly.
A MAP details the schedule of the time slots within the MAP interval. Generally, it specifies time slots within which a given CM should transmit its waiting packet on the upstream channel, i.e., a grant in response to a time-slot request. The MAP also defines time slots in which needy CMs can transmit their time-slot requests on a contention basis, and time slots that are to be used for other signaling and control purposes. In addition, a MAP also contains an “Acknowledgement” for each successful time-slot request that has not yet been granted time slots. In this disclosure, the set of time slots within a MAP interval that are dedicated to request contentions will be called “contention region” while the remaining time slots within the interval “grant region.” These terms “grant region” and “contention region” have similar meanings with respect to maps or map intervals of non-DOCSIS reservation based TDMA packet transmission systems.
Transmitting a time-slot request typically uses exactly one time slot, while a grant, which is needed to transmit one data packet, typically uses many time slots. Since grants are scheduled by the upstream scheduler, packet collisions are avoided. However, time slot requests sent over the upstream channel during the defined contention region may collide with other time slot requests if two or more CMs select the same time slot for transmission of their time slot requests. In that case, the CMs schedules the retransmission of their lost time-slot requests upon receiving a subsequent MAP in which the CM expects, but does not receive either a grant or an acknowledgement for the previous time slot request. While the basic rescheduling policy is specified by DOCSIS, certain key parameter settings are left to the designer of upstream scheduler.